The present invention relates to an image reading apparatus having a fluorescent lamp, more particularly to an image reading apparatus capable of controlling a quality of obtained output image so as to be improved in accordance with a change of an intensity of light radiated from the fluorescent lamp caused by a so-called secular change as well as a so-called shading along an extending direction of the fluorescent lamp.
Conventionally, as a light source for irradiating a desired original of an image reading apparatus, a fluorescent lamp has been widely used. However, a light amount radiated from a surface of the fluorescent lamp significantly depends on both a voltage being applied thereto and a temperature thereat. An image being read is affected by a change of the light amount thereof. Thereby, a density of the original cannot be precisely read, resulting in decreasing an image quality. To compensate the change of the light amount, a method in which the temperature on the surface of the lamp is controlled by means of a heater and a fan has been used. An another method in which the change of the light amount by the heat generation after the fluorescent lamp is turned on is measured in advance has been also used. At the time, the data relating to the light amount along the lengthwise direction are stored in a memory. The read data in accordance with image data are compensated by data being stored in the memory.
Further, as a compensation of the light amount in the lengthwise direction of the fluorescent lamp, i.e., shading compensation, another method in which the data relating to the light amount along the lengthwise direction has been stored in the memory and then the light amount is compensated in accordance with the data being stored.
However, to keep the temperature on the fluorescent lamp on a constant level during reading the image, it is necessary to provide a large scale head retaining unit or cooling unit for controlling the temperature of the surface of the fluorescent lamp so as to be kept in constant, thereby increasing a production cost and a size of the apparatus.
Moreover, the method in which the change of the light amount of the fluorescent lamp has been stored in the memory and then compensated is not proper from a view of a so-called secular change of the fluorescent lamp.
In addition, since it is necessary to provide a unit required for the shading compensation other than the unit required for the above operation, the structure of the apparatus becomes complicated.